Brain tumors are the cause of death in approximately 13,000 people in the U.S. every year, and these tumors represent the most common type of solid neoplasms in children. Recent studies have demonstrated that inducible tryptophan oxidation is an important mechanism of modulation of tumor cell proliferation and immuno-resistance, mainly via the immuno-modulatory enzyme indoleamine 2,3-dioxygenase (IDO), the rate-limiting step of the kynurenine pathway. Our preliminary studies using positron emission tomography (PET) with the tracer 1-[11C]methyl-L-tryptophan (AMT) showed differential increase of uptake and metabolism of AMT in various brain tumors, and expression of IDO in resected tumor tissue. These data suggest that accumulation of AMT in these tumors is related to increased metabolism of tryptophan via the kynurenine pathway. This application will address both basic and clinical aspects of the role of tryptophan in brain tumors in children and adults. We will perform quantitative PET studies to measure in vivo uptake and metabolism of AMT in various brain tumors and correlate these findings with tumor proliferative index, IDO immunoreactivity, and multidrug-resistance (MDR) proteins in resected tumor tissues. Three aims are proposed: (i) To establish that AMT PET can differentiate brain tumors from non-tumorous lesions preoperatively, and detect residual or recurrent tumors after initial treatment, as well as differentiate between recurrent tumors and radiation necrosis. (ii) To differentiate among various types of brain tumors based on measures of transport and metabolic trapping of AMT. (iii) To determine the relationship between kinetic parameters of AMT derived from PET imaging, and tumor proliferative index, IDO, and multidrug-resistance protein expression derived from histological assay of tumor tissue. These studies will elucidate mechanisms of abnormal uptake and metabolism of tryptophan and their role in the biological behavior of brain tumors using an innovative approach of combination of in vivo molecular imaging with AMT PET and in vitro tumor tissue studies. From a clinical perspective, our studies will establish the use of AMT PET imaging of primary and residual/recurrent brain tumors, which continue to pose a major challenge for accurate diagnosis. The findings will provide new insight into mechanisms of abnormal tryptophan uptake and metabolism in brain tumors with the potential of developing new strategies using pharmacological treatment of brain tumors by targeting tryptophan metabolism and MDR proteins. This project combines PET imaging of brain tumors with the tracer 1-[11C]methyl-L-tryptophan and in vitro analysis of the expression of the immuno-modulatory enzyme indoleamine 2,3-dioxygenase (IDO) as well as various multidrug-resistance (MDR) proteins in resected brain tumor tissues. The findings will provide improved diagnosis of primary and recurrent brain tumors and also will give new insights into mechanisms of abnormal tryptophan metabolism in brain tumors with the potential of developing new strategies using pharmacological treatment of brain tumors by targeting tryptophan metabolism and MDR proteins.